1. Field
The present disclosure generally relates to the field of radio-frequency communication devices, and more particularly, to systems and methods for controlling power in a multiple standard mobile transmitter.
2. Description of the Related Art
As the designs of portable radio-frequency (RF) communication devices, such as cellular telephones, personal digital assistants (PDAs), WiFi transceivers, and other mobile communication devices evolve, it is desirable to have such devices be capable of adjusting transmitted power accurately over a relatively wide dynamic range. For example, in the emerging markets of 3G/3.9G, linear systems such as those that communicate in accordance with standards such as WCDMA, WiMAX, EUTRAN-LTE, and other non-constant envelope modulation methodologies, the requirements for those standards for accurate transmitted power control continue to present challenges.
In mobile communication systems, power control can be mandated to ensure that the respective power levels of communication signals arriving at a base station from various mobile devices are relatively the same. To accomplish this goal, the base station can continuously monitor the received signal power from each mobile device communicating with the base station. The base station can then direct each mobile device to adjust the transmit power level depending upon one or more factors, such as its distance, data rate change, and/or channel condition.
The presence of time-varying signal information (e.g., modulation) on the envelope of the transmitted signal can create a conflict in design requirements for setting the transmitted signal power and accurately controlling the same during a signal transmission or burst in WCDMA and LTE communication systems. The first conflict can arise from a requirement to maintain average power step accuracy (in some cases the power step accuracy must be maintained to within as little as ⅛ dB). To meet this requirement, the control loop should be relatively fast to not adversely affect the average output power level. However, if the control loop is too fast, the control loop can strip information from the signal envelope, introducing an error in the transmitted signal. To minimize the impact of such errors, especially for high peak-to-average ratio modulation formats, the bandwidth of the control loop can be kept low. It is possible to apply or switch between a control loop having a bandwidth that enables a fast response and a bandwidth that permits a relatively slower response, but it takes time to ensure that the average achieved target power when operating in the fast response mode is accurate so that a switch to the slow response mode would not occur when the instantaneous error in the transmit power is substantial. Such a switch can result in a significant error in the transmitted power for an undesirable time period.
Still another problem can arise when the modulation format or data pattern changes. While the average signal power over an entire transmission or burst may not change, the average power during some part of the transmission could be off by more than the required power setting tolerance. If such a condition occurs when the control loop is operating in a fast response mode (e.g., with a wide bandwidth), the control loop could set the output power to an incorrect target value.
Conventional transmitter architectures generally apply analog power control schemes by adjusting and combining elements in a transmitter chain in an effort to generate a continuous and controllable transmitted output power. Such transmitter chains, to be successful, use several continuously variable gain elements. The performance of the elements as well as the accuracy of the transition between them can be established by design, calibration information, compensation elements, and information regarding the supply voltage, temperature, etc. Such a design can be extremely challenging and can require significant calibration effort to adjust for production tolerances and temperature variation over a relatively wide range of operational temperatures for each of the independent and overlapping gain/attenuation stages.